Palaeomagnetism of the Lower Ordovician Orthoceras Limestone, St. Petersburg, and a revised drift history for Baltica in the early Palaeozoic

Abstract

Palaeomagnetic investigation of Lower Ordovician limestone in the vicinity of St. Petersburg yields a pole position at latitude 34.7°N, longitude 59.1°E (dp/dm=5.7°/6.4°). A probable primary remanence origin is supported by the presence of a field reversal. The limestone carries one other remanent magnetization component associated with a Mesozoic remagnetization event. An apparent polar wander path is compiled for Baltica including the new result, ranging in age from Vendian to Cretaceous. Ages of the published Lower to mid‐Palaeozoic palaeomagnetic pole positions are adjusted in accordance with the timescale of Tucker & McKerrow (1995). The new Arenig result is the oldest of a series of Ordovician and Silurian palaeomagnetic pole positions from limestones in the Baltic region. There are no data to constrain apparent polar wander for the Tremadoc, Cambrian and latest Vendian. If the Fen Complex results, previously taken to be Vendian in age (c. 565 Ma), are reinterpreted as Permian remagnetizations, an Early Ordovician–Cambrian–Vendian cusp in the polar wander path for Baltica is eliminated. The apparent polar wander curve might then traverse directly from poles for Vendian dykes on the Kola peninsula (c. 580 Ma) towards our new Arenig pole (c. 480 Ma). The consequence of this change in terms of the motion of Baltica in Cambrian times is to reduce significantly a rotational component of movement. The new Arenig pole extends knowledge of Ordovician apparent polar wander an increment back in time and confirms the palaeolatitude and orientation of Baltica in some published palaeogeographies. Exclusion of the Fen Complex result places Baltica in mid‐ to high southerly latitudes at the dawn of the Palaeozoic, consistent with faunal and sedimentological evidence but at variance with some earlier palaeomagnetic reconstructions.

Two competing Paleomagnetic directions in the Late Vendian: New data for the SW Region of the Siberian Platform

“…They are reported in [Popov et al, 2002;Torsvik et al, 1996]. As demonstrated in the latter paper, all of the poles, except for two of them, (see Table 3 and Figure 18 in our paper) fit in the Permian-Triassic interval of the Baltic APWP [Smethurst et al, 1998b]. This fact combined with the absence of baked contact tests for these paleomagnetic determinations (all obtained using igneous rocks), as well as the regional Permian-Triassic remagnetization widespread in the Baltic territory [Shatsillo and Shipunov, 2005], allow one to date the poles as PermoTriassic.…”

“…[61] It is somewhat more difficult to correlate the B1 component with the Early Cambrian pole of the platform (Figure 13), because the position of the latter is a subject of hot discussion, for example, see [Pisarevsky et al, 1997;Smethurst et al, 1998a]. However, even in this case the pole of the B1 component resides at some distance from the Early Cambrian poles proposed earlier [Kirschvink and Rozanov, 1984;Pavlov et al, 2004;Pisarevsky et al,, 1997], this providing a basis for the early formation of the B1 component.…”

“…The "Winter Coast" poles [Popov et al, 2002] and those of the "Alnon Complex" [Piper, 1981] (see Figure 18 and Table 3 in this paper) reside, irrespective of the polarity choice, at considerable distances from the Baltic APWP, being, in turn, far spaced from one another, although the rocks from which they were determined have about the same age. [Smethurst et al, 1998b].…”

“…Still somewhat problematic is the determination of the polarities of these poles since the most substantiated postVendian pole being the Early Ordovician one [Smethurst et al, 1998b].…”

Investigating the duration and termination of the Early Paleozoic Moyero Reversed Polarity Superchron: Middle Ordovician paleomagnetism from Estonia

“…Ordovician time and better resolve the known temporal boundaries of the superchron (Plado et al, 2010;Smethurst et al, 1998). In terms of paleomagnetics, however, the site is nonideal.…”